Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, PR China.
Tree Physiol. 2013 Jun;33(6):579-89. doi: 10.1093/treephys/tpt040.
Tree roots are highly heterogeneous in form and function. Previous studies revealed that fine root respiration was related to root morphology, tissue nitrogen (N) concentration and temperature, and varied with both soil depth and season. The underlying mechanisms governing the relationship between root respiration and root morphology, chemistry and anatomy along the root branch order have not been addressed. Here, we examined these relationships of the first- to fifth-order roots for near surface roots (0-10 cm) of 22-year-old larch (Larix gmelinii L.) and ash (Fraxinus mandshurica L.) plantations. Root respiration rate at 18 °C was measured by gas phase O2 electrodes across the first five branching order roots (the distal roots numbered as first order) at three times of the year. Root parameters of root diameter, specific root length (SRL), tissue N concentration, total non-structural carbohydrates (starch and soluble sugar) concentration (TNC), cortical thickness and stele diameter were also measured concurrently. With increasing root order, root diameter, TNC and the ratio of root TNC to tissue N concentration increased, while the SRL, tissue N concentration and cortical proportion decreased. Root respiration rate also monotonically decreased with increasing root order in both species. Cortical tissue (including exodermis, cortical parenchyma and endodermis) was present in the first three order roots, and cross sections of the cortex for the first-order root accounted for 68% (larch) and 86% (ash) of the total cross section of the root. Root respiration was closely related to root traits such as diameter, SRL, tissue N concentration, root TNC : tissue N ratio and stele-to-root diameter proportion among the first five orders, which explained up to 81-94% of variation in the rate of root respiration for larch and up to 83-93% for ash. These results suggest that the systematic variations of root respiration rate within tree fine root system are possibly due to the changes of tissue N concentration and anatomical structure along root branch orders in both tree species, which provide deeper understanding in the mechanism of how root traits affect root respiration in woody plants.
树木根系在形态和功能上具有高度的异质性。先前的研究表明，细根呼吸与根系形态、组织氮（N）浓度和温度有关，并随土壤深度和季节而变化。然而，根系呼吸与根分支顺序中根形态、化学和解剖结构之间关系的潜在机制尚未得到解决。在这里，我们研究了 22 年生落叶松（Larix gmelinii L.）和水曲柳（Fraxinus mandshurica L.）人工林近地表（0-10cm）根系第一至第五分支顺序的这些关系。在一年中的三个时间点，通过气相 O2 电极测量了 18°C 时第一至第五分支顺序根（远端根编号为第一级）的根呼吸速率。同时测量了根直径、比根长（SRL）、组织 N 浓度、总非结构性碳水化合物（淀粉和可溶性糖）浓度（TNC）、皮层厚度和中柱直径等根参数。随着根序的增加，根直径、TNC 和根 TNC 与组织 N 浓度的比值增加，而 SRL、组织 N 浓度和皮层比例减少。在两个树种中，根呼吸速率也随着根序的增加而单调下降。皮质组织（包括外皮层、皮质薄壁组织和内皮层）存在于第一至第三级根中，一级根的皮质横截面积占根总横截面积的 68%（落叶松）和 86%（水曲柳）。在第一至第五级中，根呼吸与根直径、SRL、组织 N 浓度、根 TNC：组织 N 比以及中柱与根直径比等根性状密切相关，解释了落叶松根呼吸速率变化的 81-94%，解释了水曲柳根呼吸速率变化的 83-93%。这些结果表明，树木细根系统内根呼吸速率的系统变化可能是由于组织 N 浓度和两种树种根分支顺序的解剖结构变化所致，这为了解根性状如何影响木本植物根呼吸提供了更深入的认识。
